(A) Field of the Invention
The present disclosure relates to a semiconductor light-emitting device, and more particularly, to a semiconductor light-emitting device with improved light-extraction efficiency.
(B) Description of the Related Art
Semiconductor light-emitting devices such as light-emitting diodes (LED) are widely used in traffic lights, vehicle electronics, LCD backing lights, and general illumination. In the light-emitting diode an n-type semiconductor layer, a light-emitting region and a p-type semiconductor layer are essentially made to grow over a substrate to form a layered structure, and the electrodes are formed over the p-type semiconductor layer and over the n-type semiconductor layer. Light is generated through the recombination of holes and electrons that have been injected through the semiconductor layers to the light-emitting region, and then emitted through a light transmitting electrode over the p-type semiconductor layer or from the substrate. The material used for preparing the visible light-emitting diode includes the III-V compound such as AlGaInP for green, yellow, orange or red light-emitting diodes, and GaN for blue or ultraviolet light-emitting diodes, wherein the GaN light-emitting diode is formed over the sapphire substrate.
Extracting the light beams generated by the light-emitting layer to the outside of the light-emitting device is one important aspect to be improved in the semiconductor light-emitting device. Researchers use a transparent electrode in the conventional light-emitting device to prevent the upward light beams generated by the light-emitting layer from being blocked over the propagation path to the outside of the light-emitting device, or use a reflection layer to reflect the downward light beams generated by the light-emitting layer back to the top of the light-emitting device. However, in addition to the upward light beams and downward light beams, the light-emitting layer also emits light beams in other directions, and a portion of the light beams are reflected internally into the light-emitting device due to the total reflection effect. Consequently, the light beams may be adsorbed by the light-emitting layer, rather than propagating to the outside of the light-emitting device.
TW 561632 discloses a semiconductor light-emitting device having at least one recess and/or protruding portion over the surface portion of a substrate. The recess and/or protruding portion has a shape that prevents crystal defects from occurring in semiconductor layers. In addition, TW 536841 discloses a semiconductor light-emitting element having an undulation formed over the surface of a first layer (substrate), and a second layer having a refractory index different from that of the first layer grown to fill the undulation. Furthermore, a first crystal may be grown in an undulated shape over a crystal layer, which is the foundation of crystal growth. After such undulated refractory interface is formed, a semiconductor crystal layer having a refractory index different from that of the first layer is laminated thereon.